Failure root-cause analysis of end-ring torsional resonances and bar breakages in fabricated-cage induction motors

This work shows experimental evidences of abnormal end-ring torsional vibrations in fabricated cage induction motors and their role as root cause of bar breakages. A case-study of cage torsional resonances in railway traction motors is presented, which led to bar breakages with a peculiar failure pattern. A particular cage resonant mode (the second natural mode with both the rings vibrating in phase-opposition with respect to the rotor iron stack) is shown as responsible of excessive bar bending, fatigue, and bar fractures, through direct measurements of ring-stack oscillations in working motors. Cage torsions were excited by large harmonic torques due to inverter supply in the case considered. The mechanism of resonance-induced bar solicitations and fatigue is discussed, and the difference of failure pattern with respect to more common bar breakage mechanisms (overloads, heavy start-ups, etc.) is evidenced. Some theoretical models for modal frequency calculation are presented, which may help to identify the origin of vibrations and the fault root-cause in faulted drives. Dynamic simulations of the drive have been carried out for airgap torque calculation and bar length optimization.

I. Introduction

Mechanical resonances in induction motor drives have long been studied due to their detrimental effects such as disturbing noise [1], excessive vibrations [2], and even damage of mechanical and electrical parts [3]. The literature covers both line-fed fixed-frequency drives (FFDs) [2] and inverter-fed variable-frequency drives (VFDs) [4]. Motor resonances can be classified based on the resonant part/assembly as [5]: –

Rotordynamic (shaft lateral and torsional vibrations)

–

(frame, stator core, foundations, supports, etc.)

–

(fans, bearings, teeth, terminal boxes, etc.).